Solution of tetramethylammonium hydroxide in water and a process for preparing the solution

ABSTRACT

A solution of tetramethylammonium hydroxide in water comprises a surfactant component and a hydrotropic component. A process for preparing a solution for developing an exposed photoresist comprises the steps: preparation of a solution of tetramethylammonium hydroxide in water; addition of a surfactant component; and addition of a hydrotropic component.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a solution of tetramethylammonium hydroxide inwater with a surfactant component, and to a process for preparing asolution for developing an exposed photoresist.

In wafer production, a photoresist mask is applied to the wafer, exposedand then developed using a developer liquid. A conventional developer isbased on a 2.5% strength solution of tetramethylammonium hydroxide(TMAH) in water, comprising a small amount of added surfactant. Thesurfactant is able to reduce the interfacial tension of the developersolution to an appropriate, defined value.

The amount of developer solution required in a wafer factory isconsiderable: the development of a photoresist mask on a single waferrequires about 50 ml of developer solution. Currently, the production ofa finished wafer comprises about 20 lithography steps which means thatit is also necessary to carry out 20 development steps. A wafer factoryproducing 10,000 wafers per week requires approximately 10,000 liters ofdeveloper liquid a week. The transport of the developer liquid to thefactory thus gives rise to considerable costs.

It has been suggested that a more highly concentrated TMAH solution (forexample 25% strength) be transported to the factory and diluted in thefactory itself to give the desired final concentration. The deionized,high-purity water required for this purpose is available in all waferfactories. A reduction of transport costs would be achieved by thismeasure.

There is, however, a practical problem in that the surfactant does notdissolve in such a concentrated developer solution at standardtemperature. The surfactant precipitates in the concentrated solution,i.e. it forms insoluble aggregates, which may form deposits on containerwalls. For this reason it is not possible to transport the surfactant ina concentrated TMAH solution.

It would be possible in principle to transport the developer solution tothe factory in concentrated form without surfactant and add thesurfactant only after diluting the developer solution to its final useconcentration (approximately 2.5%). However, this procedure wouldinvolve transporting the surfactant to the factory separately inprediluted form and require an extra addition step in addition todiluting with water, which extra step is risky in terms of purityrequirements. However, the essential disadvantage of this method is thatis is usually not possible in a wafer factory to monitor and control theamount of admixed surfactant in a sufficiently exact manner. Thesedisadvantages are so serious that this procedure is not used inpractice.

SUMMARY OF THE INVENTION

The first object is achieved, according to the invention, in that thesolution further comprises a hydrotropic component.

The object of the present invention is to provide a developer solutionwhich reduces the costs associated with the development step in waferproduction. It is another object of the present invention to provide aprocess for preparing such a solution.

The first object is achieved according to the invention by thecharacterizing features of Claim 1.

The solution according to the invention has the advantage that it can beboth in a concentrated form which is particularly suitable for transportpurposes, and in a ready-to-use, low-concentration form.

In the concentrated form, for example at a TMAH concentration of 5-40%and in particular of 25%, the effect of the hydrotropic component isthat the surfactant component is soluble even at these increased TMAHconcentrations. It is thus possible to transport the TMAH solution ofthe invention in its concentrated form, correspondingly reducing theamount of solution to be transported and thus the transport costs.

The novel ready-to-use developer solution, having a TMAH concentrationof approximately 2,5%, can be obtained by simple dilution of theconcentrated TMAH solution with deionized, high-purity water. It wasfound that the hydrotropic component does not impair the developmentbehavior of the solution.

The hydrotropic component preferably consists of cumenesulfonic acid,tetramethylammonium cumenesulfonate, tetramethylammoniumxylenesulfonate, tetramethylammonium toluenesulfonate,tetramethylammonium phenolsulfonate and/or tetramethylammoniumbenzenesulfonate.

The second object of the present invention is achieved, according to theinvention, by adding a hydrotropic component.

For the preparation of a concentrated novel solution intended fortransport, the novel process is characterized in that step b) comprisesadding the surfactant component in a concentration above the cloud pointand step c) comprises adding the hydrotropic component in an amountsufficient to solubilize the surfactant component added in step b). Thesurfactant and the hydrotrope can be added in any sequence.

Advantageous embodiments of the invention are specified in the dependentclaims.

The invention will now be described by way of example with reference toillustrative embodiments.

EXAMPLES

The invention will now be described by way of example with reference toillustrative embodiments.

In a first illustrative embodiment, 100 g of a 25% strength TMAHsolution are admixed with 1 g of random n-dodecanol pentaethoxylateCH₃—(CH₂—)₁₀CH₂—O—(CH₂—CH₂—O—)₅H which acts as a surfactant. The tensideprecipitates and a cloudy solution is formed with stirring. 1.2 g ofcumenesulfonic acid solubilizer (hydrotrope) are then added. Withstirring, the solution becomes clear at room temperature. The solutionhas a cloud point, which is referred to as Krafft point and is istemperature-dependent, of 33° C. This means that the solution is stable,in its concentrated form, up to a temperature of 33° C.

If 1.7 g of cumenesulfonic acid are added instead of 1.2 g ofcumenesulfonic acid, the cloud point is raised to 55° C.

A ready-to-use developer solution is prepared by 10-fold dilution of thesolution with water.

In a second illustrative embodiment, 100 g of a 25% strength TMAHsolution are admixed with 1 g of random nonylphenol octadecaethoxylateCH₃—(CH₂—)₉C₆H₄—O—(CH₂—CH₂—O—)₁₈H which acts as a surfactant. Thetenside precipitates and a cloudy solution is formed with stirring. 1.25g of cumenesulfonic 30 acid solubilizer (hydrotrope) are then added.With stirring, the solution becomes. The solution has a cloud point of35° C.

If 1.70 g of tetramethylammonium cumenesulfonate(CH₃)₄—N+—SO₃—C₆H₄—CH—(CH₃)₂ are added in the form of a concentratedissolved in water instead of 1.25 g of cumenesulfonic acid, thesolution likewise has a cloud point of 35° C.

A ready-to-use developer solution is again prepared by 10-fold dilutionof the solution with water. The behavior of this solution in thedevelopment process is not impaired by the hydrotrope added. In the caseof tetramethylammonium cumenesulfonate as hydrotrope, this is due to thefact that the alkyl group on the aromatic ring of thetetramethylammonium cumenesulfonate is too short to exhibit any surfaceactivity in a 2.5% strength TMAH solution.

All above described illustrative embodiments have in common that theaddition of a hydrotrope in a sufficient amount reliably prevents phaseseparation in a concentrated TMAH solution comprising a surfactantcomponent, which makes it possible to keep the surfactant in a stableand clear solution even in the concentrated TMAH solutions (for example25% strength). This allows the preparation of the ready-to-use developersolution by on-site dilution of the concentrated TMAH solution in thewafer factory which leads to a significant reduction in transport costs.

What is claimed is:
 1. A concentrated developer solution, adapted fordilution with water to a use concentration, comprising: a base componentof water, a component of tetramethylammonium hydroxide, a surfactantcomponent, and a hydrotropic component in the solution, wherein saidhydrotropic component comprises at least one material selected from thegroup consisting of cumenesulfonic acid, tetramethylammoniumcumenesulfonate, tetramethylammonium xylenesulfonate,tetramethylammonium phenolsulfonate, tetramethylammoniumtoluenesulfonate, and tetramethylammonium benzenesulfonate.
 2. Thesolution according to claim 1, wherein the concentration oftetramethylammonium hydroxide in said solution is a 5-40%.
 3. Thesolution according to claim 1, wherein the concentration oftetramethylammonium hydroxide in said solution is substantially a 25%.4. A developer solution comprising a solution according to claim 1 andadditional water to dilute said solution to a 2-5% strength solution oftetramethylammonium hydroxide in water.
 5. The solution according toclaim 1, having a cloud point of 33° C. minimum.
 6. The solutionaccording to claim 1, wherein said surfactant component is nonionic. 7.The solution according to claim 1, wherein the concentration of saidsurfactant component is 1%.
 8. The solution according to claim 1,wherein the concentration of said hydrotropic component is in the rangefrom 1.2% to 1.7%.
 9. The solution according to claim 1, wherein saidhydrotropic component is cumenesulfonic acid.
 10. The solution accordingto claim 1, wherein said hydrotropic component is tetramethylammoniumcumenesulfonate.
 11. The solution according to claim 1, wherein saidsolution is substantially free of inorganic ions.
 12. A process forpreparing a concentrated developer solution, adapted for dilution withwater to a use concentration, which comprises the following steps: a)preparing a solution of tetramethylammonium hydroxide in water; b)adding a surfactant component to the solution; and c) adding ahydrotropic component to the solution, wherein the hydrotropic componentincludes at least one material selected from the group consisting ofcumenesulfonic acid, tetramethylammonium cumenesulfonate,tetramethylammonium xylenesulfonate, tetramethylammoniumphenol-sulfonate and tetramethylammonium benzene-sulfonate.
 13. Theprocess according to claim 12, wherein step b) comprises adding thesurfactant component in a concentration above a cloud point, and step c)comprises adding the hydrotropic component in an amount sufficient forsolubilizing the surfactant component added in step b).
 14. The processaccording to claim 12, wherein step a) comprises forming a 5-40%strength solution of tetramethylammonium hydroxide in water.
 15. Theprocess according to claim 12, wherein step a) comprises forming asubstantially 25% strength solution of tetramethylammonium hydroxide inwater.
 16. The process according to claim 12, which further comprises:d) diluting the solution with deionized water to achieve atetramethylammonium hydroxide concentration rendering the solutionsuitable as a developer solution in a photoresist development step. 17.The process according to claim 16, which comprises forming the dilutedsolution with a tetramethylammonium hydroxide concentration of 2-5 inwater.
 18. The process according to claim 16, which comprises formingthe diluted solution with a tetramethylammonium hydroxide concentrationof approximately 2.5 in water.
 19. The process according to claim 12,which comprises selecting the surfactant component from the groupconsisting of dodecanol pentaethoxylate and nonylphenoloctadecaethoxylate.
 20. The developer solution according to claim 4,wherein said solution is substantially a 2.5% strength solution oftetramethyl-ammonium hydroxide in water.
 21. The solution according toclaim 6, wherein said surfactant component includes at least one ofdodecanol pentaethoxylate and nonylphenol octadecaethoxylate.